Chromium selenide cluster complexes [Cr6Se8(PEt3)(6)] (1) and [Cr6Se8(H)(PEt3)(6)] (2) were prepared by the reaction of anhydrous chromium dichloride, Na2Sex, and triethylphosphine in methanol. A similar procedure except for the use of NaSxH in place of Na2Sex gave a sulfide cluster complex [Cr6S8(H)(PEt3)(6)] (3) The partly deuterated derivative [Cr6S8(D)(0.8)(H)(0.2)(PEt3)(6)] (4) was prepared for comparison. An extra hydrogen atom in 2-4 has been confirmed by FAB mass spectra. The reactivity and structural studies have indicated that it is at the center of the Cr-6 octahedral cluster. The molecular structures of 1 and 2 have been determined by X-ray structure analyses. Crystallographic data: [Cr6Se8(PEt3)(6)] (1) (226 K), triclinic, P (1) over bar, a = 12.887(2) Angstrom, b = 25.332(7) Angstrom, c = 12.061(3) Angstrom, alpha = 111.23(2)degrees, beta = 108.25(2)degrees, gamma = 109.86(2)degrees, Z = 2; [Cr6Se8(H)(PEt3)(6)]. 2THF (2) (293 K), rhombohedral, R (3) over bar, a = 17.384(4) Angstrom, c = 19.768(4) Angstrom, Z = 3. The Cr-Cr bond distances of 2 are 0.13-0.16 Angstrom shorter than those of 1, indicative of bonding interaction between the insterstitial hydrogen and the six chromium atoms in 2. The number of cluster valence electrons for 1 is 20 while that for 2 and 3 is 21. Magnetic measurements (2-330 K for 1, 2.5-330 K for 2, and 4.5-330 K for 3) have shown that the number of unpaired spins in the ground state for 1 is zero while that for 2 and 3 is one.